Electric discharge machining method and device

ABSTRACT

An electric discharge machining device, in which a pulse voltage is repeatedly applied between electrodes, which are an electrode and a workpiece to be machined, to cause electric discharge between the electrodes and thereby machine the workpiece. The electric discharge machining device includes a switching device (4) connected between a power source and the machining section, for controlling energy supplied by the power source; interelectrode impedance detecting apparatus (10) for detecting an interelectrode impedance affected by the variation in place of electric discharge which takes place between the electrodes; discharge detecting apparatus for detecting when electric discharge takes place between the electrodes; and a control device (18). The control device utilizes the output of the interelectrode impedance detecting apparatus to calculate a power source internal impedance to obtain a desired no-load voltage, and a power source internal impedance to obtain a desired current at the detection of electric discharge to thereby control the switching device. As a result, the discharge machining operation is stably carried out, the machined surface is uniform in surface roughness, and for a desired surface roughness the highest machining speed can be employed.

DESCRIPTION

1. TECHNICAL FIELD

This invention relates to an electric discharge machining method and anelectric discharge machining device for use in an electric dischargemachine using an electrically conductive solution as its machiningsolution.

2. TECHNICAL BACKGROUND

FIG. 9 is a circuit diagram showing a machining power source accordingto a conventional electric discharge machining method disclosed, forinstance, by Japanese Patent Application (OPI) No. 85826/1985 (the term"OPI" as used herein means an "unexamined published application"). InFIG. 9, reference numeral 1 designates a machining electrode; 2, aworkpiece; 3, a second DC power source; 4 and 26, power transistors; 5and 25, current limiting resistors connected to the emitters of thepower transistors 4 and 26, respectively; 9, discharge detecting meansfor detecting when electric discharge takes place between theelectrodes, namely, the electrode 1 and the workpiece 2; 12, switchingmeans; 13, a second drive circuit for driving the power transistor 4; 6and 15, diodes for preventing reverse current; 18, a first drive circuitfor driving the power transistor 26; and 19, a first DC power source.The switching means 12 control the first and second drive circuits 13and 18.

The electrical characteristics of the circuit in the case where anelectrically conductive solution is used will be described.

If, in this case, the machining electrode 1 and the workpiece 2 are flatplates arranged in parallel to each other, then the interelectrodeimpedance Rgap can be represented by the following equation (1) asindicated in FIG. 10: ##EQU1## where ρ is the specific resistance (Ω cm)of the machining solution, l is the distance (cm) between theelectrodes, and S is the confronting area (cm²) between the electrodes.

When the power transistor 4 is turned on by the second drive circuit 13,a voltage Vgopen as shown in the part (a) of FIG. 6 is developed betweenthe electrodes, namely, the machining electrode 1 and the workpiece 2before electric discharge takes place therebetween. In this case,according to Ohm's law, the voltage Vgopen is: ##EQU2## where R_(M) isthe current limiting resistance, and E is the DC supply voltage.

Hereinafter, the voltage Vgopen will be referred to as "a no-loadvoltage", and the interelectrode voltage provided after electricdischarge will be referred to as "an arc voltage Vgarc", whenapplicable.

The current flowing between the electrodes is as follows: That is, ifthe total current supplied by the power source is represented by I, withrespect to the interelectrode impedance Rgap an electrolytic currentflowing according to Ohm's law at the application of a no-load voltageis represented by I_(Eopen), and that during electric discharge isrepresented by I_(Earc), and a discharge current in the electricdischarge is represented by Id, then

Before electric discharge

    I=I.sub.Eopen                                              ( 3)

During electric discharge

    I=Id+I.sub.Earc                                            ( 4)

where ##EQU3##

As is clear from equation (1), the interelectrode impedance Rgap isdecreased as the resistivity ρ of the machining solution or theinterelectrode distance l is decreased, and as the confronting area Sbetween the electrodes is increased. Furthermore, as is apparent fromequation (2), the no-load voltage Vgopen decreases as the interelectrodeimpedance Rgap decreases. When the no-load voltage becomes lower thanthe arc voltage Vgarc, then no electric discharge will take placebetween the electrodes; that is, the workpiece cannot be machined.Therefore, in the discharge-machining of a large area, the resistivity ρof the machining solution should be maintained high to some extent. Forthis purpose, the resistivity ρ is controlled by using ion exchangeresin for instance.

On the other hand, when it is required to decrease the discharge currentId, the resistance R_(M) of the current limiting resistor 5 should beset to a large value; however, in this case, the no-load voltage Vgopenis decreased, thus making it difficult for electric discharge to takeplace; that is, the machining efficiency is lowered. For the purpose ofeliminating the abovedescribed difficulty, the following electricdischarge machining method is employed for an electric discharge machineusing an electrically conductive machining solution.

As was described above, FIG. 9 shows the discharge machining powersource circuit. In FIG. 9 two current circuits are connected in parallelto the electrode 1 and the workpiece 2. The machining current (ordischarge current) is supplied by the circuit comprising the powertransistor 4, the second DC power source 3, the current limitingresistor 5, and the diode 6. The power transistor 4 is driven by thesecond drive circuit 13. Before electric discharge takes place, thecircuit comprising the power transistor 26 driven by the first drivecircuit 18, the current limiting resistor 25, the diode 15, and thefirst DC power source 19, applies a larger current between the electrode1 and the workpiece 2. That is, the current is made larger than thatwhich flows during the discharge period, whereby the interelectrodevoltage under no load is increased to cause electric discharge betweenthe electrodes with ease. When electric discharge takes place betweenthe electrodes, the discharge detecting means 9 detects the occurrenceof electric discharge, and the switching means 12 applies a signal tothe first drive circuit 18, so that the power transistor 26 is turnedoff, and the discharge current is therefore supplied from the second DCpower source 3 only.

In this case, the resistance R_(M) of the resistor 5 has been set to avalue with which a discharge current corresponding to a desired surfaceroughness and machining speed can be obtained, and the resistance R_(S)of the resistor 25 has been set a value with which, with the resistanceR_(M) taken into account, a current necessary for the provision of ano-load voltage high enough to start electric discharge can be obtained.

The no-load voltage Vgopen thus obtained is: ##EQU4## where E₁ is the DCsupply voltage on the side of the first drive circuit, and E₂ is the DCsupply voltage on the side of the second drive circuit.

As is apparent from the above description, in the conventional dischargemachining method, the power source internal impedance is switched.However, in the case where the interelectrode impedance Rgap is obtainedfrom equation (1) for calculation of the power source internalimpedance, there are the following problems:

(1) The confronting surfaces of the electrode 1 and the workpiece 2 arenot always flat. Therefore, it is not always possible to insert thedistance between the electrodes in "l" in equation (1) as it is.

(2) As shown in FIG. 11, depending on a machining pattern, theconfronting area between the electrodes is varied as the machiningoperation makes progress. The variation of the interelectrode impedanceRgap due to that variation of the confronting area is too large to beneglected.

(3) The resistivity of the electrically conductive machining solutionchanges with progress of the machining operation, and, as shown in FIG.12, has different values in the machining solution tank, in themachining bath, and in the discharge gap. Thus, it is difficult tomeasure the resistivity of the machining solution in the discharge gap.

Thus, it is difficult to calculate the interelectrode impedance Rgap byusing the interelectrode distance l, the confronting area S between theelectrodes, and the resistivity ρ of the machining solution, while theinterelectrode impedance changes with progress of the machiningoperation. If, with the interelectrode impedance Rgap changing withprogress of the machining operation, the power source internal impedanceis not corrected, then the following difficulty will be involved:

As the electrolytic current I_(Earc) flowing during the electricdischarge changes, the discharge current Id is changed as is apparentfrom equation (4). As a result, for the aimed surface roughness, it isimpossible to maintain the highest machining speed or to maintain themachined surface uniform in roughness.

Accordingly, an object of this invention is to eliminate theabove-described difficulties. More specifically, an object of theinvention is to provide an electric discharge machining method in whicha discharge machining operation is stably carried out, and a powersource internal impedance is automatically changed and set, whereby themachined surface is uniform in roughness while the interelectrodeimpedance Rgap is being affected by the interelectrode distance l,confronting area S, and resistivity ρ which change with progress of thedischarge machining operation, and in which for an aimed surfaceroughness, the highest machining speed can be maintained at all times.

In the electric discharge machining method according to the invention,an interelectrode impedance is detected, and according to theinterelectrode impedance thus detected, a power source internalimpedance with which a desired no-load voltage is obtained and a powersource internal impedance with which a desired discharge current isobtained are calculated, and those internal impedances are set up for adischarge-machining power source circuit.

In the electric discharge machining method of the invention, theinterelectrode impedance detected is so utilized that, during theno-load period which elapses from the application of voltage between theelectrodes until electric discharge takes place therebetween, a powersource internal impedance with which a no-load voltage equal to orhigher than the discharge starting voltage is supplied is calculated,and after the discharge starts, a power source internal impedance withwhich a desired current is supplied is controlled to a predeterminedvalue.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a machining power source employed inone example of an electric discharge machining method according to thisinvention.

FIGS. 2 through 5 are circuit diagrams showing machining power sourcesin other examples of the electric discharge machining method of theinvention, respectively.

FIG. 6 is a waveform diagram showing interelectrode voltages andinterelectrode currents.

FIG. 7 is a graphical representation indicating arc voltage withdischarge current.

FIG. 8 is an explanatory diagram showing a memory for storing the latestinterelectrode impedance.

FIG. 9 is a circuit diagram showing a machining power source employed ina conventional electric discharge machining method.

FIG. 10 is an explanatory diagram showing the arrangement of electrodeswith an electrically conductive machining solution.

FIG. 11 is an explanatory diagram showing increase of the confrontingarea of an electrode intricate in configuration with progress of thedischarge machining operation.

FIG. 12 is an explanatory diagram showing the variation in resistivityof a machining solution.

BEST MODE FOR CARRYING OUT THE INVENTION

On embodiment of this invention will be described with reference to theaccompanying drawings.

In FIGS. 1 through 5, reference numeral 1 designates a machiningelectrode; 2, a workpiece; 3, a machining DC power source; 4, a group ofpower transistors 4-1, 4-2, . . . , and 4-n (hereinafter referred to as"a power transistor group 4", when applicable); 5, a group of currentlimiting resistors 5-1, 5-2, . . . and 5-n which are connected to theemitters of the power transistors 4-1 through 4-n, respectively(hereinafter referred to as "a current limiting resistor group", whenapplicable); 6 and 15, diodes for preventing reverse current; 7, adetecting DC power source; 8, a resistor for limiting the output currentof the detecting DC power source 7; 9, discharge detecting means fordetecting when electric discharge takes place between the electrodes;10, detecting means for detecting an interelectrode impedance Rgap; 11,arithmetic means for calculating a power source internal impedancesuitable for the interelectrode impedance Rgap detected by the detectingmeans 10; and 12, switching means for determining the on-off pattern ofthe power transistor group 4, i.e., an switching output, from the resultof calculation provided by the arithmetic means 11, and temporaritystoring a plurality of such patterns.

Further in FIGS. 1 through 5, reference numeral 13 designates a drivecircuit which can turn on desired ones, in combination, of the powertransistors 4-1 through 4-n; 14, memory means for storing a power sourceinternal impedance calculated by the arithmetic means 11; 16, decodemeans for determining patterns of combination of the resistors in thecurrent limiting resistor group 5 and the power transistors connectedthereto; 17, an oscillator for transmitting the output signal of thedischarge detecting means 9 to the switching means 12; 18, a powersource control circuit comprising the above-described elements 11, 12,14, 16 and 17; 19, a detecting resistor switching circuit for switchingthe current limiting resistors 8; and 24, a voltage divider for dividingthe voltage appearing across the interelectrodes.

The operations of the above-described circuit elements will bedescribed.

The detecting means 10 directly measures the interelectrode impedanceRgap according to the following methods: These methods can eliminate theabove-described difficulties, because they can directly measure theinterelectrode impedance Rgap independently of the interelectrodedistance l, the interelectrode confronting area S, and the specificresistance of the machining solution in equation (1) above.

The first method is as follows: While the machining DC power source 3 isat rest, a detecting voltage which is the same in polarity as that ofthe machining DC power source 3 is applied between the electrodes, andthe voltage developed between the electrodes is utilized to calculatethe inter-electrode impedance Rgap.

That is, as shown in FIGS. 1 or 2, the detecting DC power source 7 isconnected through the current limiting resistor or resistors 8 to theelectrodes. If, during detection, electric discharge takes place betweenthe electrodes, it is impossible to achieve the detection. Therefore,the interelectrode voltage Vga should be so determined that it may notexceed the arc voltage. Therefore, if the voltage Va of the detecting DCpower source 7 is set lower than the arc voltage Vgarc as shown in FIG.1, then control can be achieved with ease. Even in the case ofVa >Vgarc, the supply voltage Va can be set lower than the arc voltageVgarc, if, as shown in FIG. 2, the voltage Vgap detected by thedetecting means 10 is processed by the arithmetic means 11, and thecurrent limiting resistor or resistors are selected by the detectingresistor switching circuit 19. Furthermore, when the current limitingresistors are suitable selected to provide the best resistance, then theaccuracy of detection of the voltage Vgap detected by the detectingmeans 10 can be increased.

The supply voltage Va, the resistance Ra, the measured interelectrodevoltage Vga, and the measured interelectrode impedance Rgap has thefollowing relation: ##EQU5##

Therefore,the interelectrode impedance Rgap is: ##EQU6##

The interelectrode impedance Rgap can be obtained according to thefollowing method, too:

That is, in FIG. 3, voltage is applied between the electrodes by themachining power source 3, and the interelectrode voltage Vgopen providedduring the no-load period in which no discharge takes place is utilizedto calculate the interelectrode impedance Rgap. This will be describedin more detail. The supply voltage E, the interelectrode voltage Vgopen,and the interelectrode impedance Rgap have the following relation:##EQU7## where Rx is the power source internal resistance at theapplication of no-load voltage.

Therefore, the interelectrode impedance Rgap is: ##EQU8## Theinterelectrode voltage Vgopen may be inputted through an A/D converter,as a digital value, into the detecting means 10.

The method is advantageous in that it is unnecessary to use theinterelectrode impedance Rgap detecting DC power source Va (7), and thecurrent limiting resistor Ra (8).

On the other hand, the method is disadvantageous in that, in the casewhere electric discharge occurs immediately after the application ofvoltage between the electrode by the machining power source, theinterelectrode voltage Vgopen under no load cannot be detected, andtherefore the interelectrode impedance Rgap cannot be calculated.

A third method of obtaining the interelectrode impedance Rgap is asfollows: As shown in FIG. 4, even in the pause period, some of the powertransistors 4-1 through 4-n are turned on, and the resistance of thecurrent limiting resistors (5) is set to a sufficiently large value sothat the interelectrode voltage is lower than the arc voltage, and thevoltage Vgz provided between the electrodes is utilized for calculationof the interelectrode impedance. If the power source internal impedanceprovided during the pause period is represented by Rz, then thefollowing relation is established by the supply voltage E, theinterelectrode voltage Vgz, and the interelectrode impedance Rgap:##EQU9##

Therefore, the interelectrode impedance Rgap is: ##EQU10##

The above-described method is advantageous in that the interelectrodeimpedance Rgap, the detecting DC power source Va (7), and the currentlimiting resistor Ra (8) are unnecessary. Furthermore, in the method,during the pause period the interelectrode voltage Vgz is detected forcalculation of the interelectrode impedance Rgap, and thereforesimilarly as in the above-described second method the interelectrodeimpedance Rgap can be calculated even in the case where discharge takesplace immediately after the application of voltage, and there is nono-load period.

A fourth method of obtaining the interelectrode impedance Rgap is asfollows: In the fourth method as shown in FIG. 5, during the pauseperiod of the machining power source 3, the detecting DC power source 7whose voltage is lower than the arc voltage Vgarc is used to apply thedetecting voltage between the electrodes which is the same in polarityas that of the machining power source 3, and the voltage developedbetween the electrodes is utilized for calculation of the interelectrodeimpedance Rgap. The specific feature of the method resides in that thecurrent limiting resistor group 5 of the machining power source 3 isutilized as the current limiting resistance for the detecting DC powersource 7. Therefore, when compared with the above-described "firstmethod of obtaining the interelectrode impedance Rgap", the method isadvantageous in that it is unnecessary to additionally provide currentlimiting resistors 8 for the detecting DC power source 7.

The arithmetic means 11 utilizes the interelectrode impedance measuredby the detecting means 10 to calculate power source internal impedancesseparately according to the following conditions:

Firstly, under the no-load condition which is provided after applicationof the voltage until electric discharge occurs between the electrodes,the power source internal impedance Rx providing a no-load voltageVgopen high enough to permit electric discharge to take place iscalculated. Upon detection of the occurrence of electric dischargebetween the electrodes, the power source internal impedance Rx isswitched over to a power source internal impedance Ry to obtain adesired discharge current Id during the discharge; however, since theswitching elements 4 such as power transistors have switching delaytime, during that period the following instantaneous current I_(DPeak)flows between the electrodes as shown in the part (b) of FIG. 6. Thus,the method suffers from a difficulty that a current larger than thedesired discharge current Id flows as the instantaneous currentI_(Dpeak), thus roughening the machining surface. ##EQU11##

In order to obtain a high no-load voltage Vgopen, the power sourceinternal impedance Rx should be small; however, it is set to a suitablevalue with which the instantaneous current I_(Dpeak) will not roughenthe machining surface.

In this case, the relation between the no-load voltage Vgopen and theinterelectrode impedance Rgap and the power source internal impedance Rxis: ##EQU12## where E is the voltage of the DC power source 3.

That is, the aimed no-load voltage Vgopen and the measuredinterelectrode Rgap are utilized to calculate the power source internalimpedance Rx according the abovedescribed equation 11.

Secondly, during electric discharge, an electrolytic current, which doesnot contribute to the machining of the workpiece, changes with theinterelectrode impedance Rgap, and therefore in order to control thedischarge current Id which is necessary for the machining of theworkpiece, the total current I which is the sum of the discharge currentId and the electrolytic current IEarc is controlled according to theinterelectrode impedance Rgap.

During the discharge, the electrolytic current I_(Earc) and thedischarge current Id can be represented by the following equations:##EQU13##

In the above-described equation, Ry is the power source internalimpedance which is a composite resistance determined by selectivelycombining the current limiting resistors. It has been confirmed throughexperiment that the arc voltage Vgarc depends on the discharge currentId. By using the measured interelectrode impedance Rgap, the desireddischarge current value Id, and the arc voltage Vgarc, the power sourceinternal impedance Ry is: ##EQU14##

The power source internal impedances Rx and Ry calculated by thearithmetic means 11 are applied to the memory means 14. In the case ofthe (above-described third) method of applying a voltage between theelectrodes during the pause period to obtain the interelectrodeimpedance Rgap, a power source internal impedance Rz with which, duringthe pause period, the interelectrode voltage is set to lower than thearc voltage is also applied to the memory means 14. The switching means12 operates to select Rx during the no-load period, Ry during thedischarge, and Rz during the pause period. For the purpose of realizingthe internal impedances Rx, Ry and Rx, the decode means 16 determinesthose to be used in combination of the current limiting resistors (5),and selects the power transistors connected to the resistors thusdetermined, and provides power transistor combination patterns forinstance as follows: ##EQU15## where a₁, a₂, . . . and a_(n) are 0 or 1.

In the equation, Rx may be replaced by Ry or Rz. And in the equation,R₁, R₂, . . . and R_(n) are the resistances of the current limitingresistors 5-1, 5-2, . . . and 5-n, respectively. The sequence {a_(n) }is so determined that the following is established: ##EQU16##

The aimed no-load voltage Vgopen should be equal to at least the arcvoltage Vgarc; however, it may be any value higher than the arc voltage.

When a target value has been determined for the no-load voltage Vgopen,the following two control methods may be considered: in the firstcontrol method the no-load voltage is made to be coincident with thetarget value, and in the second control method, in the case where valueshigher than the target value can be readily obtained, the no-loadvoltage is not particularly reduced to the target value. That is, in thecase where the electrode area S is small, and the interelectrodeimpedance Rgap is sufficiently high, the second control method may betaken.

However, discharge detection by the discharge detecting means 9 is, ingeneral, achieved by comparing the interelectrode voltage with areference discharge voltage, and therefore the no-load voltage dependson the time delay of discharge detection. Accordingly, the no-loadvoltage should be maintained as constant as possible, and it ispreferable that control is so made by the arithmetic means 11 that theno-load voltage is constant.

By the above-described method, the interelectrode impedance Rgap can becalculated with the detecting means 10, and the data calculated can beoutputted every voltage pulse.

The arithmetic means 11 utilizes the interelectrode impedance Rgap,which is outputted by the detecting means 10 every voltage pulse, tocalculate the power source internal impedance.

In obtaining the power source internal impedance from the interelectrodeimpedance outputted by the detecting means 10, the interelectrodeimpedance Rgap data calculated may be processed by the followingmethods:

In a first one of the methods, the interelectrode impedance Rgapoutputted every pulse is utilized for calculation of the internalimpedance at the next pulse. More specifically, in the method, theinterelectrode impedance at the preceding pulse is utilized fordetermination of the internal impedance when the present pulse isapplied. Thus, the method can respond to the change in condition betweenthe electrodes.

On the other hand, during an electric discharge machining operation, theelectrodes may be short-circuited or almost short-circuited temporarilyby a turbulance such as accumulation of chips.

Under this condition, the interelectrode voltage is zero or almost zero.That is, zero or a value close to zero can be inserted in Rga, Rgopenand Rgz in equations (8), (9) and (10). Therefore, as Rgap approacheszero, Rx and Ry calculated by equations (11) and (12) also approacheszero. This means that the machining DC power source 3 supplies a largecurrent between the electrodes through a small resistance. Therefore, inthis case, electric discharge takes place with large energy, as a resultof which the machining surface is roughened more than intended.

In order to eliminate the above-described difficulty, a second method isprovided. In the second method, the average value Rgap-m of n latestones of interelectrode impedance Rgap data is obtained, and it isutilized for calculation of the internal impedance. Let us consider amemory having addresses 1 through n as shown in FIG. 8. Rgap data arestored in the memory in such a manner that newly inputted data is storedin the address 1 always, the old data in the address 1 is shifted to thenext address. Thus, the memory stores n latest Rgap data at all times.By using the contents of the memory, the arithmetic means 11 performsthe following calculation: ##EQU17## The average value Rgap-m thusobtained is utilized for calculation of the power source internalimpedance.

The following third method may be realized by using the memory describedabove with respect to the second method: That is, in the third method,of the n latest interelectrode impedances Rgap stored in the memory, thelargest Rgap is employed as a typical value. Setting the value n to asuitable value can eliminate the difficulty that the value Rgap of zeroor nearly zero which occurs when the electrodes are short-circuited oralmost short-circuited.

Next, the drive circuit 13 for the power transistor group 4 decodes apower transistor select combination pattern supplied from the switchingmeans 12, to apply a signal to the signal lines connected to the basesof the concerned power transistors to turn on the latter on.

As was described above, in response to the variation of interelectrodeimpedance Rgap, the power source internal impedance is changed accordingto the following conditions of each voltage pulse:

(1) When the voltage is applied between the electrodes after the pauseperiod, the power source internal impedance 11 is set to Rx calculatedfrom equation (11),

(2) When the discharge detecting means 9 detects when electric dischargetakes places between the electrodes, the detection signal is applied tothe switching means 12, so that the internal impedance Rx is changedover to Ry calculated from equation (12), and

(3) When the desired voltage pulse period has passed, all the powertransistors are turned off, so that the pause period is provided.

The machining operation is advanced by repeated carrying out theabove-described operations.

As is apparent from the above description, the electric dischargemachining method of the invention is so designed that the interelectrodeimpedance is detected from its variation attributing to the change inphase of the electric discharge between the electrodes, and the datathereof thus detected are utilized for calculation of the power sourceinternal impedance. Therefore, with the method of the invention, thebest no-load voltage and the desired machining current can be readilyobtained. Accordingly, the resultant discharge-machined surface has auniform surface roughness which is defined univocally by settingmachining conditions, the machining operation being stable, andfurthermore the large area machining operation and the finishingoperation, which cannot be achieved by the conventionalelectric-discharge machining method, can be carried out. Thus, theelectric discharge machining method of the invention will reduce themanufacturing cost. The effects in practical used of the method shouldbe highly appreciated.

INDUSTRIAL APPLICABILITY

While the preferred embodiment of the invention has been described withreference to the general electric discharge machine, the technicalconcept of the invention is similarly applicable to wire cut typeelectric discharge machines.

We claim:
 1. An electric discharge machining method in which an electrically conductive solution is used as a machining solution, a pulse voltage is repeatedly applied between electrodes which are an electrode and a workpiece to be machined, to cause electric discharge to take place between said electrodes, thereby to machine said workpiece; in whichan interelectrode impedance Rgap defined by the distance between said electrodes, the confronting area thereof, and the specific resistivity of said electrically conductive solution is detected, during a no-load period which is provided after application of said voltage between said electrodes until electric discharge takes place between said electrodes, a power source internal impedance Rx with which a no-load voltage equal to or higher than a discharge starting voltage is supplied is set, after detection of the occurrence of electric discharge between said electrodes, a power source internal impedance Ry with which a desired machining current is supplied is set, and said machining current is allowed to flow for a predetermined period of time, and with a predetermined pause period, said series of operations are repeatedly carried out under control.
 2. An electric discharge machining method as claimed in claim 1, in which said power source internal impedance Rx and Ry are calculated by using said detected interelectrode impedance Rgap.
 3. An electric discharge machining method as claimed in claim 2, in which during a discharge voltage pause period a detecting DC power source whose output voltage is equal to or lower than an arc voltage is operated to apply a voltage, which is the same in polarity as that of a machining DC power source, between said electrodes through a current limiting resistor, and the resultant value of voltage or current provided between said electrodes are utilized for calculation of said interelectrode impedance Rgap.
 4. An electric discharge machining method as claimed in claim 3, in which said interelectrode impedance Rgap is calculated from the following equation: ##EQU18## where Va is the detecting power source voltage, Vga is the interelectrode voltage, and Ra is the resistance of a resistor (8).
 5. An electric discharge machining method as claimed in claim 2, in which during a no-load period which elapses from application of a voltage between said electrodes by a machining DC power source until electric discharge takes place between said electrodes, the resultant value of voltage or current provided between said electrodes is utilized for calculation of said interelectrode impedance.
 6. An electric discharge machining method as claimed in claim 5, in which said interelectrode impedance Rgap is calculated from the following equation: ##EQU19## where Vgopen is the interelectrode voltage during the no-load period, E is the supply voltage, and Rx is the power source internal impedance at the application of the no-load voltage.
 7. An electric discharge machining method as claimed in claim 2, in which during a discharge voltage pause period, a machining DC power source applies a voltage between said electrodes through a current limiting resistor, and the resultant value of voltage or current provided between said electrodes is utilized for calculation of said interelectrode impedance Rgap.
 8. An electric discharge machining method as claimed in claim 7, in which said interelectrode impedance is calculated from the following equation: ##EQU20## where E is the supply voltage, Vgz is the interelectrode voltage, and Rz is the power source internal impedance during the pause period.
 9. An electric discharge machining method as claimed in claim 1, in which a no-load voltage set during a no-load period which elapses from application of a voltage between said electrodes until electric discharge takes place between said electrodes has a predetermined value equal to or higher than a discharge starting voltage.
 10. An electric discharge machining method as claimed in claim 2, in which said power source internal impedances Rx and Ry are calculated from the following equations, respectively: ##EQU21## wherein Rgap is the interelectrode impedance, Vgopen is the no-load voltage, Vgarc is the arc voltage, E is the supply voltage, and Id is the discharge current.
 11. An electric discharge machining method as claimed in claim 2, in which an interelectrode impedance Rgap outputted every pulse is utilized for calculation of said power source internal impedances Rx and Ry at the next pulse.
 12. An electric discharge machining method as claimed in claim 2, in which the average value Rgap-m of n most recent interelectrode impedance Rgap data (where n≧2) is utilized for calculation of said power source internal impedance Rx and Ry.
 13. An electric discharge machining method as claimed in claim 2, in which the largest of interelectrode impedance Rgap data is employed as a typical value for calculation of said power source internal impedances Rx and Ry.
 14. An electric discharge machining device in which a pulse voltage is repeatedly applied to an interelectrode gap which is formed by an electrode and a workpiece to be machined, to cause electric discharge between said electrode and workpiece to machine said workpiece, which comprises:switching means for controlling machining energy, which is connected between a power source and a machining section; interelectrode impedance detecting means for detecting an interelectrode impedance affected by the variation in phase of electric discharge which takes place between said electrodes; discharge detecting means for detecting when electric discharge takes place between said electrodes; and control means which, according to an output of said interelectrode impedance detecting means, calculates a power source internal impedance to obtain a desired no-load voltage, and a power source internal impedance to obtain a desired current at the detection of electric discharge thereby to control said switching means.
 15. An electric discharge machining device as claimed in claim 14, in which said switching means comprises a plurality of power transistors parallel-connected.
 16. An electric discharge machining device as claimed in claim 15, which further comprises a detecting DC power source connected between said electrodes in such a manner that said detecting DC power source is the same in polarity as said machining power source.
 17. An electric discharge machining device as claimed in claim 16, in which a variable limiting resistor is connected in series to said detecting DC power source.
 18. An electric discharge machining device as claimed in claim 14 which said control means comprises:arithmetic means for calculating a power source internal impedance according to an interelectrode impedance detected; and means for controlling an output according to the result of calculation of said arithmetic means which is applied to said switching means. 